Acoustic liner and method of forming an acoustic liner

ABSTRACT

An acoustic liner includes a first face sheet, a second face sheet spaced from the first face sheet, and a plurality of sidewalls extending between the first face sheet and the second face sheet. The plurality of sidewalls defines a plurality of cells. Each cell of the plurality of cells defines a cavity between the first face sheet and the second face sheet. A bulk absorber is disposed within at least one cell of the plurality of cells. The bulk absorber further defines the cavity of the at least one cell of the plurality of cells. The first face sheet defines a plurality of apertures extending through a thickness of the first face sheet. Each aperture of the plurality of apertures is aligned with a respective cell of the plurality of cells.

This application is a continuation of U.S. patent application Ser. No.16/240,360 filed Jan. 4, 2019, which is hereby incorporated herein byreference in its entirety.

BACKGROUND 1. Technical Field

This disclosure relates generally to noise attenuation structures, andmore particularly to acoustic liners for aircraft gas turbine engines.

2. Background Information

A gas turbine engine may include an acoustic liner for attenuating noisegenerated during engine operation. These acoustic liners may generallyhave a sandwich structure formed by face sheets enclosing a cellularhoneycomb-type inner structure. Under some circumstances, liquidsencountered during engine operation, such as water, de-icer fluid, fuel,etc., may interact with the acoustic liner thereby interfering with thenoise attenuating characteristics of the liner. Further, the threat ofliquid interaction with acoustic liners may preclude the use of certainmaterials in liners which would enhance liner performance. Conventionalsolutions to these problems can be expensive and result in excessiveacoustic liner weight and/or volume or, in some cases, can limit thefrequency range of sound absorption. Accordingly, a need exists for animproved acoustic liner.

SUMMARY

According to an embodiment of the present disclosure, an acoustic linerincludes a first face sheet, a second face sheet spaced from the firstface sheet, and a plurality of sidewalls extending between the firstface sheet and the second face sheet. The plurality of sidewalls definesa plurality of cells. Each cell of the plurality of cells defines acavity between the first face sheet and the second face sheet. A bulkabsorber is disposed within at least one cell of the plurality of cells.The bulk absorber further defines the cavity of the at least one cell ofthe plurality of cells. The first face sheet defines a plurality ofapertures extending through ha thickness of the first face sheet. Eachaperture of the plurality of apertures is aligned with a respective cellof the plurality of cells.

In the alternative or additionally thereto, in the foregoing embodiment,the acoustic liner further includes a membrane disposed proximate thefirst face sheet.

In the alternative or additionally thereto, in the foregoing embodiment,a portion of the membrane is free of mechanical constraint, the portioncorresponding to a respective span of an adjacent aperture of theplurality of apertures.

In the alternative or additionally thereto, in the foregoing embodiment,the membrane has an areal density of 5-100 g/m².

In the alternative or additionally thereto, in the foregoing embodiment,the membrane is a non-porous membrane.

In the alternative or additionally thereto, in the foregoing embodiment,the bulk absorber is disposed adjacent the first face sheet and thecavity is further defined between the bulk absorber and the second facesheet.

In the alternative or additionally thereto, in the foregoing embodiment,the membrane includes at least one of a hydrophobic and an oleophobicmaterial.

In the alternative or additionally thereto, in the foregoing embodiment,the membrane is disposed opposite the first face sheet with respect tothe plurality of cells.

In the alternative or additionally thereto, in the foregoing embodiment,the membrane is a membrane coating disposed on a surface of the bulkabsorber proximate the first face sheet.

In the alternative or additionally thereto, in the foregoing embodiment,the bulk absorber includes at least one of a hydrophobic and anoleophobic material.

In the alternative or additionally thereto, in the foregoing embodiment,the bulk absorber includes a first portion having a first porosity and asecond portion having a second porosity, greater than the firstporosity.

In the alternative or additionally thereto, in the foregoing embodiment,the first portion is proximate the first face sheet and the secondportion is proximate the second face sheet.

In the alternative or additionally thereto, in the foregoing embodiment,the bulk absorber includes a gradated porosity extending from the firstportion to the second portion.

In the alternative or additionally thereto, in the foregoing embodiment,a length of the bulk absorber is less than half of a length of theplurality of cells.

According to another embodiment of the present disclosure, a gas turbineengine includes at least one acoustic liner is disposed on an internalsurface of the gas turbine engine. The at least one acoustic linerincludes a first face sheet, a second face sheet spaced from the firstface sheet, and a plurality of sidewalls extending between the firstface sheet and the second face sheet. The plurality of sidewalls definesa plurality of cells. Each cell of the plurality of cells defines acavity between the first face sheet and the second face sheet. A bulkabsorber is disposed within at least one cell of the plurality of cells.The bulk absorber further defines the cavity of the at least one cell ofthe plurality of cells. The first face sheet defines a plurality ofapertures extending through ha thickness of the first face sheet. Eachaperture of the plurality of apertures is aligned with a respective cellof the plurality of cells.

In the alternative or additionally thereto, in the foregoing embodiment,the at least one acoustic liner further includes a membrane disposedproximate the first face sheet.

In the alternative or additionally thereto, in the foregoing embodiment,the membrane is a membrane coating disposed on a surface of the bulkabsorber proximate the first face sheet.

In the alternative or additionally thereto, in the foregoing embodiment,the bulk absorber includes at least one of a hydrophobic and anoleophobic material.

According to another embodiment of the present disclosure, a method offorming an acoustic liner includes providing a base including a solidface sheet and a plurality of side walls extending from the solid facesheet. The plurality of side walls define a plurality of cells. At leastone of the plurality of cells is at least partially filled with a bulkabsorber.

In the alternative or additionally thereto, in the foregoing embodiment,a surface of the bulk absorber, opposite the solid face sheet, is coatedwith a membrane.

The present disclosure, and all its aspects, embodiments and advantagesassociated therewith will become more readily apparent in view of thedetailed description provided below, including the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an aircraft.

FIG. 2 is a cross-sectional, side view of a gas turbine engine.

FIG. 3 is a perspective, cut-away view of an exemplary acoustic liner.

FIG. 4 is cross-sectional side view of the acoustic liner of FIG. 3.

FIG. 5A is cross-sectional side view of an exemplary acoustic liner.

FIG. 5B is cross-sectional side view of an exemplary acoustic liner.

FIG. 5C is cross-sectional side view of an exemplary acoustic liner.

FIG. 6 illustrates a method of forming an acoustic liner.

DETAILED DESCRIPTION

It is noted that various connections are set forth between elements inthe following description and in the drawings. It is noted that theseconnections are general and, unless specified otherwise, may be director indirect and that this specification is not intended to be limitingin this respect. A coupling between two or more entities may refer to adirect connection or an indirect connection. An indirect connection mayincorporate one or more intervening entities. It is further noted thatvarious method or process steps for embodiments of the presentdisclosure are described in the following description and drawings. Thedescription may present the method and/or process steps as a particularsequence. However, to the extent that the method or process does notrely on the particular order of steps set forth herein, the method orprocess should not be limited to the particular sequence of stepsdescribed. As one of ordinary skill in the art would appreciate, othersequences of steps may be possible. Therefore, the particular order ofthe steps set forth in the description should not be construed as alimitation.

Referring to FIGS. 1 and 2, an aircraft 1000 includes a gas turbineengine 10 mounted to, for example, a wing 1002 of the aircraft 1000. Thegas turbine engine 10 is centered about a longitudinal axis 12 andincludes a nacelle 14 forming an outer perimeter of the gas turbineengine 10. The nacelle 14 has an intake portion 16. A fan section 18 isdisposed downstream of the intake portion 16. An annular air bypass 20is disposed downstream of the fan section 18 and generally definedbetween an inner wall 22 and an outer wall 24 (e.g., an inner barrel ofthe nacelle 14). An engine core 26 is located radially inward of theinner wall 22. An exhaust nozzle 30 is located downstream of the enginecore 26. At least a portion of air entering the intake portion 16 of thenacelle 14 (i.e., bypass flowpath A1) will pass through the fan section18 and subsequently through the air bypass 20.

Referring to FIG. 2, an acoustic liner 28 may be carried, by an internalsurface of the gas turbine engine 10, for example, a radially inwardfacing wall of the intake portion 16 of the nacelle 14, the inner and/orouter walls 22, 24, the exhaust nozzle 30, or other suitable surfaces ofthe gas turbine engine 10, to attenuate noise in the vicinity of theacoustic liner 28. It should be further understood that the presentdisclosure is not limited to use in gas turbine engines or aircraft andmay be applied to any other vehicle, application, or environment wherenoise suppression, and particularly low frequency noise suppression, isdesirable.

Referring to FIGS. 3 and 4, the acoustic liner 28 may include aperforated face sheet 32 and a solid face sheet 34 spaced from theperforated face sheet 32. For example, in some embodiments, theperforated face sheet 32 and the solid face sheet 34 may be orientedsubstantially parallel to one another. Each of the perforated face sheet32 and the solid face sheet 34 include an inner face 36, 38 and an outerface 40, 42, respectively. A plurality of sidewalls 44 extend betweenthe perforated face sheet 32 and the solid face sheet 34, therebydefining a plurality of cells 46 between the perforated face sheet 32and the solid face sheet 34. The plurality of cells 46 has a length L1.Each cell of the plurality of cells 46 defines a cavity 48 (i.e., avoid). The perforated face sheet 32 defines a plurality of apertures 50extending through a thickness T1 of the perforated face sheet 32. Eachaperture of the plurality of apertures 50 is aligned with a respectivecell of the plurality of cells 46.

The acoustic liner 28 may define, in part, an air passage (e.g., apassage about bypass flowpath A1) of the gas turbine engine 10, forexample, the intake portion 16 of the nacelle 14, the air bypass 20,etc. Each aperture of the plurality of apertures 50 may define an airflowpath A2 between an interior and an exterior of each respective cellof the plurality of cells 46. For example, the air flowpath A2 may bedefined between the bypass flowpath A1 and each respective cell of theplurality of cells 46.

As shown in FIG. 3, the plurality of cells 46 defined by the pluralityof sidewalls 44 can be configured with a “honeycomb” structure definedby, for example, six walls of the plurality of sidewalls 44. However,aspects of the disclosed embodiments may be applied to cells havingother configurations as well.

Still referring to FIGS. 3 and 4, in operation, for example, anairstream (e.g., bypass flowpath A1) flows across the acoustic liner 28proximate the outer face 40 of the perforated face sheet 32 in ashearing direction. Air and/or noise from this airstream generallyenters each cell of the plurality of cells 46 along the air flowpath A2through each aperture of the plurality of apertures 50. In acousticliners, like those described generally above, the cells of the honeycombstructure covered by the perforated face sheet 32 form resonant cavities(e.g., Helmholtz resonance chambers) that contribute to the dissipationof incident acoustic energy by attenuating acoustic reflected wavesand/or converting acoustic energy into heat energy, such as by Helmholtzresonance.

The acoustic liner 28 may be made of any variety of materials dependingupon a particular application including metals, composites, andceramics. For example, if the acoustic liner 28 is applied to theexhaust nozzle 30 of the gas turbine engine 10, an acoustic liner madeat least in part of ceramic may be desirable to withstand exposure tohigh temperatures. In some embodiments, one or both of the face sheets32, 34 may be made from a fiber-reinforced material, a sound absorbingmaterial, or any other suitable material. The acoustic liner 28 may bemanufactured using any variety and combination of known manufacturingtechniques. In some embodiments, the acoustic liner 28 may bemanufactured using, at least in part, an additive manufacturing process.

The acoustic liner 28 may further include a membrane 52 disposedproximate the perforated face sheet 32 and configured to prevent entryof liquids (e.g., water, de-icer fluid, fuel, etc.) into an interior ofthe cells of the plurality of cells 46. For example, in someembodiments, the membrane 52 may be disposed adjacent the outer face 40of the perforated face sheet 32. The membrane 52 has a thickness T3. Insome embodiments, the membrane 52 may have a thickness T3 fromapproximately 10 microns to 500 microns. The membrane 52 may be amicroporous membrane having, for example, a porosity of 50% or greater,an average pore size between 0.1 microns and 1.0 microns, a surfaceenergy from approximately 15 mJ/m² (millijoules per square meter) to 25mJ/m², and/or an areal density of 5-100 g/m² (grams per square meter).In some embodiments, the membrane 52 may be an air-permeable membranemade from at least one of an oleophobic and a hydrophobic material,including, but not limited to a fibrous non-woven material, a wovenconstruct, or a grid pattern based on polymers or inorganic fibers suchas polyethylene, terephthalate, polyamide, carbon fiber, aramid fiber,or glass fibers, or any other suitable material. The membrane 52 mayalso be, for example, a gel processed microporous film such as SOLUPOR10PO5A membrane. In some other embodiments, the membrane 52 may be athin impermeable, nonporous polymer membrane. The non-porous membranemay be made of, for example, polyether ether ketone (PEEK), polyetherketone ketone (PEKK), polyvinyl fluoride (PVF), polyvinylidene fluoride(PVDF), ethylene tetrafluoroethylene (ETFE), polysulfone (PSU)polyethylene terephthalate (PET), or any other suitable material. Themembrane 52 may also be, for example, a lightweight scrim reinforcedPEKK film such as LAMAGUARD 10. In some embodiments, the membrane 52 maybe coated to reduce surface energy with a thin, conformal or nearlyconformal oleophobic and/or hydrophobic coating such as TEFLON AF 1601,NOVEC 2702, or any other suitable oleophobic and/or hydrophobic coating.

Referring to FIGS. 5A-5C, the acoustic liner 28 may include a bulkabsorber 54, such as but not limited to an open cell foam, a closed cellfoam, and/or a fibrous batting, disposed within at least a portion of atleast one cell of the plurality of cells 46. As used herein, open cellfoam refers to a foam in which adjacent cells are connected by openingswhich allow for the free passage of air between adjacent cells. The bulkabsorber configured as an open cell foam may be made of polyimide,melamine, aluminum, titanium, or any other suitable polymer or metal.For example, the open cell foam may be SOLIMIDE AC-530. As used herein,fibrous batting refers to an arrangement of widely spaced fibersenclosing a large volume of air. The fibrous batting may be made of, forexample, alumino-borosilicate glass fiber, polyethylene terephthalatefiber, aramid fiber, carbon fiber, or any other suitable fiber material.The fibrous batting may be, for example, DBCORE.

The bulk absorber 54 may extend substantially a width W1 of the cell.The bulk absorber 54 has a thickness T2. The bulk absorber 54 mayfurther define the cavity 48 of the at least one cell of the pluralityof cells 46 (i.e., a volume bounded by the bulk absorber 54, the solidface sheet 34, and the respective cell of the plurality of cells 46). Insome embodiments, the bulk absorber 54 may be disposed adjacent theperforated face sheet 32.

The acoustic energy of sound waves can be transmitted through membrane52, for example, by the free passage of moving air through ahighly-porous, low-flow-resistance membrane or by the free vibration ofan unconstrained non-porous membrane. The membrane 52 and the bulkabsorber 54 dissipate acoustic energy through the friction of airmolecules moving through the porous material. Acoustic energy may alsobe dissipated to a less extent by the mechanical vibration of themembrane 52 and the bulk absorber 54. These mechanisms lead to overallsound absorption.

The membrane 52 may prevent or reduce the ingress of liquids encounteredduring gas turbine engine 10 operation to the cell via air flowpath A2(see FIG. 4), and hence interaction between the liquid and the bulkabsorber 54. Interaction between liquids and the bulk absorber 54 can,for example, result in blockages of the pores of the bulk absorber 54.Such blockages can reduce the acoustic attenuating capabilities of thebulk absorber 54, and accordingly the acoustic liner 28, by preventingor reducing the flow of air through the bulk absorber 54 along airflowpath A2. In some embodiments, the membrane 52 may be free ofmechanical constraint on its inner and outer sides (i.e., the sidefacing the perforated face sheet 32 and the opposing side) along thespan of an aperture of the plurality of apertures. Such a configurationof the membrane 52 may permit unconstrained vibration of the membrane 52over the span of the aperture thereby allowing the membrane 52 to beacoustically transparent (i.e., allows transmission of the incidentenergy to the bulk absorber 54).

Referring to FIG. 5B, in some embodiments, the membrane 52 may bedisposed on a surface 56 of the bulk absorber 54 proximate theperforated face sheet 32 (i.e., the membrane 52 may be adjacent to theperforated face sheet 32 and/or disposed on a surface of the bulkabsorber 54 closest to the perforated face sheet 32). For example, themembrane 52 may be a membrane coating applied and/or adhered to thesurface 56 of the bulk absorber 54 in at least one cell of the pluralityof cells 46. In some embodiments, at least a portion of a length of themembrane 52 may be bounded by the at least one cell of the plurality ofcells 46.

Referring to FIG. 5C, in some embodiments, the bulk absorber 54 maycomprise at least one of an oleophobic and a hydrophobic material. Insuch an embodiment, inclusion of the membrane 52 in the acoustic liner28 may not be necessary.

In some embodiments, the bulk absorber 54 may include a first portion 58and a second portion 60 of the volume of the bulk absorber 54, whereinthe first portion 58 of the bulk absorber 54 has a different porositythan the second portion 60 of the bulk absorber 54. For example, in someembodiments, the first portion 58 of the bulk absorber 54 may bedisposed proximate the perforated face sheet 32 while the second portion60 of the bulk absorber 54 is disposed proximate the solid face sheet34. For further example, in some embodiments, the second portion 60 ofthe bulk absorber 54 may have a greater porosity than the first portion58 of the bulk absorber 54.

In some embodiments, the bulk absorber 54 has a gradated porositybetween the first portion 58 of the bulk absorber 54 and the secondportion 60 of the bulk absorber 54. For example, the porosity of thebulk absorber 54 may gradually transition from the first porosity of thefirst portion 58 of the bulk absorber 54 to the second porosity of thesecond portion 60 of the bulk absorber 54 along the thickness T2 of thebulk absorber 54 between the first portion 58 and the second portion 60.In some other embodiments, the bulk absorber 54 may gradually transitionfrom an open cell foam to a closed cell foam along the thickness T2 ofthe open cell foam 54.

It will be appreciated by those of ordinary skill in the art that thephysical dimensions of components of the acoustic liner 28 may bealtered or tuned to attenuate targeted resonating frequency bandwidths.For example, a length, width, height, and/or volume of one or morecavities 48 may be selected to attenuate a particular frequency orfrequency range. In some embodiments, for example, the acoustic liner 28may be tuned to suppress the relatively high-frequency bandwidthsemitted from a turbine section of the engine core 26 or may be tuned tosuppress the lower frequency bandwidths emitted from a combustor sectionof the engine core 26. The membrane 52 and/or bulk absorber thicknessT3, T2 or material selection may permit the acoustic frequency responseof the acoustic liner 28 to be tailorable to desired noise bands forabsorption. For example, in some embodiments, the thickness T2 of thebulk absorber 54 may be less than half of a length L1 of the pluralityof cells 46. In some other embodiments, the length of the bulk absorber54 may be greater than half of the length of the plurality of cells 46.

Referring to FIG. 6, a flowchart is provided illustrating a method 100of forming an acoustic liner. In step 102, a base can be provided. Thebase may include the solid face sheet 34 and the plurality of sidewalls44 extending from the solid face sheet 34 and defining the plurality ofcells 46. Each cell of the plurality of cells 46 may further define thecavity 48. In step 104, at least one cell of the plurality of cells 46may be filled, at least partially, with the open cell foam 54. While theterm “fill” or “filled” may be used, it should be understood that theopen cell foam 54 may not completely fill the corresponding cell of theplurality of cells 46. In step 106, the surface 56 of the open cell foam54, opposite the solid face sheet 34, may be coated with the membrane52. In step 108, the perforated face sheet 32 may be coupled to the base(i.e., to the plurality of cells 46 opposite the solid face sheet 34thereby forming a sandwiched configuration of the plurality of cells 46between the solid face sheet 34 and the perforated face sheet 32). Theplurality of apertures 50 of the perforated face sheet 32 may be alignedsuch that each aperture of the plurality of apertures 50 is aligned witha respective cell of the plurality of cells 46.

While various aspects of the present disclosure have been disclosed, itwill be apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible within the scope of thepresent disclosure. For example, the present disclosure as describedherein includes several aspects and embodiments that include particularfeatures. Although these particular features may be describedindividually, it is within the scope of the present disclosure that someor all of these features may be combined with any one of the aspects andremain within the scope of the present disclosure. Accordingly, thepresent disclosure is not to be restricted except in light of theattached claims and their equivalents.

What is claimed is:
 1. An acoustic liner comprising: a first face sheetincluding an inner surface, an outer surface opposite the inner surface,and a plurality of apertures, the plurality of apertures extendingthrough the first face sheet from the outer surface to the innersurface; a second face sheet spaced from the first face sheet; and aplurality of sidewalls mounted between the first face sheet and thesecond face sheet, the plurality of sidewalls forming a plurality ofcells, a first cell of the plurality of cells aligned with at least oneaperture of the plurality of apertures, the first cell forming andsurrounding a cavity adjacent the second face sheet, the first cellincluding: a membrane including a first membrane side and a secondmembrane side, the first membrane side contacting the inner surface withthe first membrane side located contiguous with the at least oneaperture; a bulk absorber including a first absorber side and a secondabsorber side, the first absorber side contacting the second membraneside, the bulk absorber further defining the cavity between the secondabsorber side and the second face sheet.
 2. The acoustic liner of claim1, wherein the membrane has an areal density of 5-100 g/m².
 3. Theacoustic liner of claim 1, wherein the membrane is a microporousmembrane.
 4. The acoustic liner of claim 1, wherein the membraneincludes one or both of a hydrophobic material and an oleophobicmaterial.
 5. The acoustic liner of claim 1, wherein the bulk absorbercomprises a first portion having a first porosity and a second portionhaving a second porosity, greater than the first porosity.
 6. Theacoustic liner of claim 5, wherein the first portion is proximate thefirst face sheet and the second portion is proximate the second facesheet.
 7. The acoustic liner of claim 5, wherein the bulk absorbercomprises a gradated porosity extending from the first portion to thesecond portion.
 8. The acoustic liner of claim 1, wherein a length ofthe bulk absorber is less than half of a length of the first cell. 9.The acoustic liner of claim 1, wherein the bulk absorber is an open cellfoam.
 10. The acoustic liner of claim 1, wherein the membrane issurrounded by sidewalls of the plurality of sidewalls which form thefirst cell.
 11. A gas turbine engine comprising: at least one acousticliner disposed on an internal surface of the gas turbine engine, the atleast one acoustic liner comprising: a first face sheet including aninner surface, an outer surface opposite the inner surface, and aplurality of apertures, the plurality of apertures extending through thefirst face sheet from the outer surface to the inner surface; a secondface sheet spaced from the first face sheet; and a plurality ofsidewalls mounted between the first face sheet and the second facesheet, the plurality of sidewalls forming a plurality of cells, a firstcell of the plurality of cells aligned with at least one aperture of theplurality of apertures, the first cell forming and surrounding a cavityadjacent the second face sheet, the first cell including: a membraneincluding a first membrane side and a second membrane side, the firstmembrane side contacting the inner surface with the first membrane sidelocated contiguous with the at least one aperture; a bulk absorberincluding a first absorber side and a second absorber side, the firstabsorber side contacting the second membrane side, the bulk absorberfurther defining the cavity between the second absorber side and thesecond face sheet.
 12. The gas turbine engine of claim 11, wherein thebulk absorber is an open cell foam.
 13. The gas turbine engine of claim12, wherein the membrane is adhered to a surface of the bulk absorber.14. The gas turbine engine of claim 12, wherein the bulk absorbercomprises a first portion having a first porosity and a second portionhaving a second porosity, greater than the first porosity.
 15. The gasturbine engine of claim 14, wherein the first portion is proximate thefirst face sheet and the second portion is proximate the second facesheet.
 16. An acoustic liner comprising: a first face sheet including aninner surface, an outer surface opposite the inner surface, and aplurality of apertures, the plurality of apertures extending through thefirst face sheet from the outer surface to the inner surface; a secondface sheet spaced from the first face sheet; and a plurality ofsidewalls mounted between the first face sheet and the second facesheet, the plurality of sidewalls defining a plurality of cells, a firstcell of the plurality of cells aligned with at least one aperture of theplurality of apertures, the first cell including: a membrane surroundedby sidewalls of the plurality of sidewalls which define the first cell,the membrane including a first membrane side and a second membrane side,the first membrane side contacting the inner surface with the firstmembrane side located contiguous with the at least one aperture; a bulkabsorber including a first absorber side and a second absorber side, thefirst absorber side contacting the second membrane side.
 17. Theacoustic liner of claim 16, wherein the membrane includes one or both ofa hydrophobic material and an oleophobic material.
 18. The acousticliner of claim 16, wherein the membrane has a thickness between 10microns and 500 microns.
 19. The acoustic liner of claim 16, wherein themembrane is a microporous membrane.
 20. The acoustic liner of claim 16,wherein the membrane includes one or both of an oleophobic and ahydrophobic coating.